This invention is related to a control for direct current motors utilizing silicon controlled rectifiers (SCR's) to supply power to the motor and in particular to a circuit for bypassing the SCR control so that full battery potential may be applied to the motor.
It is well known that the direct current supplied to a motor from a constant potential power source such as a battery may be selectively varied by controlling the average power supplied to the motor, and that a solid state SCR can be used as a switching device to repeatedly connect and disconnect the battery to and from the motor. The power supplied to the motor is determined by a ratio between the time the SCR is turned on and conducts and the time that the SCR is turned off and is non-conducting.
Increasing the ratio of on-time to off-time will increase the power delivered through the SCR to the motor. However, even if this ratio is maximum, i.e., with the SCR conducting continuously, the full power available from the battery will not be delivered to the motor since some power will be consumed by the SCR and the circuitry for turning the SCR on and off. In addition, an SCR which can remain full-on and withstand the high currents present during such operations as ramp starts is very expensive.
As a consequence, SCR motor control circuits are generally provided with bypass contacts which may be closed to shunt out the SCR control and connect the terminals of the motor directly across the battery. With the SCR control thus shorted out, the full available power from the battery can be utilized for driving the motor, and a smaller and cheaper main SCR can be used in the SCR control.
Various safeguards should be incorporated into a bypass control. For example, in vehicle propulsion systems, the switch that actuates the bypass contact is typically operated by the foot-controlled accelerator pedal, and closes when the pedal is fully depressed. In normal driving operation, the operator may inadvertently and momentarily depress the pedal fully, which will close the bypass switch, at a time when he does not wish to go into bypass mode. Accordingly, the bypass control should operate only when the operator really intends it to operate.
Further the bypass control should be automatically disabled in case the main SCR fails to conduct. If the main SCR fails to conduct, the accelerator pedal can be depressed without causing any vehicle movement. If the bypass contacts then close around a defective main SCR, full power would then be applied to a stationary vehicle and dangerous lurching will occur.
Additionally, the bypass control should be inoperative during a plugging operation, i.e., when the field has been reversed so that the motor is acting as a generator to brake the vehicle, since application of full power to the motor during braking could cause a dangerously abrupt halt of the vehicle.
Also, in the event that the bypass contacts should weld closed, the bypass control should prevent further vehicle operation until the defective contacts have been replaced.